The icon of the Media Controls Plasmoid in the System Tray is dark and it is not change with dark themes. So, you can't see it
Created attachment 113372 [details] System tray
Please confirm which dark ktheme you are using.
I use Arc Dark, but I have just tested with Brisa Dark or others and it is the same, the icon is black always
*** Bug 395619 has been marked as a duplicate of this bug. ***
Manjaro OS Plasma 5.13.1 KDE Framework: 5.47.0 Qt: 5.11.1 Kernel: 4.17 I confirm this bug. So far EVERY non-breeze theme I tried causes media icon to fallback to the square one which is only in dark color so is visible only on light panels. Breeze, Breeze Dark, Breeze Light have proper media icon. Issue appeared after updating to Plasma 5.13. All non-breeze themes use this default, round media icon and yet newest Plasma can't register it. Is it a bug or maybe a change in Plasma theming that requires from themes developers to update their themes?
Here is another thread about this problem: https://forum.manjaro.org/t/kde-breath-theme-invisible-media-control-systray-icon/51644/2 It seems that not only media icon is touched with this issue. I could easily replicate it on Neon vbox install when using any non-breeze theme.
Here is another thread of a lost user... People are confused by this "invisible" icon. https://forum.manjaro.org/t/media-reproductor-icon-not-showing-in-trey-area/52392
*** This bug has been marked as a duplicate of bug 397082 ***